This application relates to measurements of polarization of light.
The state of polarization of light is an important parameter of an optical beam in part because it affects behavior of the optical beam when interacting with an optical medium or an optical element. Various optical devices and systems can be sensitive to the state of polarization of the beam to be processed or transmitted. For example, certain coherent optical processing may require a match between the states of polarization of two separate optical beams when the two beams are superposed. For another example, a birefringent optical element may attenuate an optical signal differently when the polarization of the signal forms different angles with respect to a given principal axis of polarization of the element. An optical amplifier with a saturable gain medium may also produce a polarization-dependent gain when a polarization component with a high intensity saturates the gain medium and hence experiences an optical gain less than that of another, weaker polarization component. Furthermore, certain optical modulators may also produce different modulation depths on optical signals with different polarizations. Semiconductor electro-absorption modulators and electro-optical modulators based on birefringent crystals such as lithium niobate are examples of such modulators.
Hence, it is desirable to control the polarization of an optical signal in those and other polarization-sensitive devices and systems. To achieve such polarization control, it is essential to measure the state of polarization of the signal so that a proper polarization control can be applied in response to the measured polarization. Various polarimeters have developed to measure the state of polarization of light based on the Stokes polarization vector. Such polarimeters may be designed to split light into four different beams for measuring the Stokes vector components.
In one implementation, for example, a first beam is used to measure the total intensity of the light; second and third beams are sent through polarizers at different relative angles where the transmitted intensities are measured; and a fourth beam is sent through a phase retarder and a polarizer where the transmitted intensity is measured. The measured intensities of the four beams are then used to compute the four Stockes vector components which uniquely determine the state of polarization.
The polarization of an optical signal may not be static but dynamically vary with time in some optical systems due to fluctuations in factors such as light sources, optical components, and optical transmission media. For example, some optical fibers may be birefringent to exhibit different refractive indices for different polarizations. Typical causes for this fiber birefringence include, among others, imperfect circular cores, and unbalanced stress in a fiber along different transverse directions. Fluctuations in local a temperature and stress along a fiber line, therefore, can randomly change the axis of birefringence of the optical fiber at different locations. The polarization of light transmitting through such a fiber, therefore, may also fluctuate with time. This can also cause polarization-mode dispersion (PMD) in optical signals with two orthogonal principal polarization states.
Hence, it may also be desirable that the polarimeter operates sufficiently fast so that a polarization control mechanism can change its control in response to any variation in the input polarization of light and therefore maintain the output polarization at a desired state.
In-line optical polarimeters and techniques for calibrating such polarimeters are described. In one implementation a polarimeter integrates components in free space to enhance device performance. For example, a device may include:
an optical path in free space to transmit an input optical beam;
a first polarization-selective element in said optical path having a first reflective surface at 45 degrees with respect to said optical path to reflect a fraction of said input optical beam in the S polarization to produce a first monitor beam and to transmit the remaining input optical beam along said optical path as a first transmitted beam;
a second polarization-selective element in said optical path having a reflective surface at 45 degrees with respect to said optical path and rotated from said first reflective surface around said optical path by 45 degrees to reflect a fraction of said first transmitted beam in the S polarization to produce a second monitor beam and to transmit the remaining of said first transmitted beam along said optical path as a second transmitted beam;
a third polarization-selective element in said optical path having a reflective surface at 45 degrees with respect to said optical path and rotated from said first reflective surface around said optical path by 90 degrees to reflect a fraction of said second transmitted beam in the S polarization to produce a third monitor beam and to transmit the remaining of said second transmitted beam along said optical path as a third transmitted beam;
a quarter-wave plate in said optical path to transmit said third transmitted beam as a fourth transmitted beam;
a fourth polarization-selective element in said optical path having a reflective surface at 45 degrees with respect to said optical path and rotated from said first reflective surface around said optical path by 135 degrees to reflect a fraction of said fourth transmitted beam in the S polarization to produce a fourth monitor beam and to transmit the remaining of said fourth transmitted beam along said optical path as an output transmitted beam;
four optical detectors respectively to receive said first, said second, said third, and said fourth monitor beams to produce first, second, third, and fourth detector signals, respectively; and
a processing circuit coupled to receive said first, said second, said third, and said fourth detector signals to determine an input polarization state of said input optical beam.